1. Field of the Disclosure
The present disclosure relates to the field of temperature control, and more particularly, to maintaining a set point temperature through heating and/or cooling an electronic device or component, typically while the electronic device or component is under test.
2. Background Information
Solid state electronic devices or components, such as semiconductors, having varying performance characteristics based on temperature. Typically, for example, such electronic devices generate heat (i.e., self-heat) during operation, and thus as the internal temperature increases, the performance characteristics change. Also, solid state electronic devices may be used in different environments, possibly enduring a wide range of temperatures.
To ensure constant performance characteristics, it is desirable to maintain a relatively constant temperature of electronic devices. This is especially true when functionally testing electronic devices to ensure proper operation and compliance with design specifications. For example, an electronic device, referred to as a device under test (DUT), may undergo endurance procedures, such as short-circuit testing and burn-in testing, to observe various device characteristics. During such testing, the temperature of the DUT must be kept relatively constant at a predetermined test temperature, or set point temperature, in order for the results to be meaningful. In other words, the tester must be able to confirm that certain observed electrical characteristics are attributable to factors other than changing temperatures.
In order to maintain a constant temperature, known thermal control devices are cable of removing heat, e.g., through a heat sink, as well as adding heat, e.g., through an electric heater. A heat sink incorporates a fluid having a temperature much lower than the test temperature of the DUT. A heater is placed between the DUT and the heat sink, and power is applied to the heater to raise the temperature of the heater face, e.g., to the test temperature required for DUT testing. The heat sink offsets any excess heating, and also removes heat generated by the DUT during the testing process, to the extent this self-heating increases the device temperature beyond the test temperature. Power fluctuations may cause significant and relatively instantaneous self-heating, requiring the need for the thermal controller to quickly and accurately react to offset the unwanted increase in temperature.
However, the total amount of power that can be removed is limited by the heater itself, which has a maximum for power density (or Watt Density). For example, if a heater is capable of operating at 500 Watts, then approximately half of that power may be lost through the heat sink into the colder fluid simply to maintain the test temperature. Thus, for example, 250 Watts are required to maintain test temperature. Then, if power in the heater is reduced to zero in response to power being applied to the DUT, the maximum amount of power which can be removed from the DUT is 250 Watts. Otherwise, the heater will be unable to offset heat removed through the heat sink. This is particularly problematic in that current requirements of DUT testing have risen to 500 Watts total power and are projected to be higher in the future. Also, the heater also adds unwanted thermal resistance, adds thermal mass, induces gradients (non-thermally uniform surface) and renders an inadequate response time.
Improvements to this type of thermal controller are difficult to implement. For example, the heat sink must be appropriately balanced to the heater, which may be a disincentive for improving heat sink efficiency. That is, if the heat sink's heat removal capability is improved, e.g., by increasing fluid flow through the heat sink, reducing the fluid temperature, improving fin efficiency and/or incorporating a more effective fluid, the heater capacity would likewise need to be increased to offset the improvements in cooling capabilities and maintain the testing temperature.
Other thermal controllers are not necessarily dependent on the combination of heat sinks and heaters, but they still have functional inefficiencies. For example, Peltier devices create heat differentials from electric voltages, effectively acting as both a heat sink and a heat source. A drawback of Peltier devices, though, is that they are unable to remove significant power or to handle high power densities because the response time required to dynamically react to and remove power from an electronic device is inadequate. Therefore, the current thermal controllers do not adequately meet the demands of maintaining a constant temperature of an electronic device.
For example, FIG. 1 depicts a graph showing a typical thermal response of a conventional temperature control device, used to regulate the temperature of a DUT. The vertical axis indicates degrees in Celsius and the horizontal axis indicates passage of time, e.g., in seconds. The test temperature 102, also known as the set point temperature, represents the desired temperature at which testing is to be performed. FIG. 1 indicates set point temperature of 90 degrees Celsius. Also indicated is a fluid temperature 104, which is the temperature of fluid flowing through a heat sink, for example, which is configured to remove heat from the DUT. The fluid delta T is the difference in temperature between the test temperature 102 and the fluid temperature 104. The fluid temperature 104 depicted in the exemplary graph is approximately 30 degrees Celsius, which would result in the fluid delta T being 70 degrees Celsius. As is well known in the art, a greater fluid delta T enables a faster cooling capability of the heat sink.
As indicated, power is applied to both a heater and the DUT in the testing environment. The heater power 110 begins at a level (e.g., 500 Watts) that enables the heater to maintain a maximum temperature, indicated as 300 degrees Celsius in this example. Then, as device power 112 is incrementally added to the DUT to perform the desired testing, the heater power 110 is correspondingly decreased, reducing the heater temperature to compensate for the increase the device temperature 108. However, once the heater power 110 is reduced to zero, and the device temperature 108 continues to rise, the effects of the fluid temperature 104 of the heat sink is no longer sufficient to cool the DUT to maintain the test temperature 102, indicated by the depicted increase in the device temperature 108 after the heater power 110 is at zero. The testing system may thus become “broken” well before the device power 112 has reached the level required for appropriate testing.
As discussed above, increasing the fluid delta T of the heat sink is not particularly effective or efficient. This is because the heater power 110 (and thus the heater temperature) must be increased to compensate for the lower fluid temperature 104, particularly when the device power 112 is a lower levels. Much of this increase in heater power 110 is negated by the increased fluid delta T.